1. Field of the Invention
The present invention relates to an abnormality determination device for determining existence or nonexistence of an abnormality in a cylinder pressure sensor that senses cylinder pressure of an internal combustion engine.
2. Description of Related Art
An engine (internal combustion engine) combusts a mixture gas in a combustion chamber and works by depressing a piston with combustion pressure. Therefore, the combustion pressure is a parameter directly indicating a combustion state. If the combustion state is grasped, estimation of a fuel injection amount or combustion temperature, detection of knocking, detection of a peak position of the combustion pressure, detection of misfire and the like are enabled. Therefore, the combustion pressure is used in various kinds of engine control such as control of fuel injection timing or an air fuel ratio.
A general and practical method of sensing the combustion pressure provides a cylinder pressure sensor in a cylinder and senses the cylinder pressure of the engine, and furthermore, the cylinder pressure as of the combustion (combustion pressure) based on an output of the cylinder pressure sensor. However, it is not always true that the output of the cylinder pressure sensor is a proper value. For example, drift can be caused in the output signal due to external factors such as change in temperature such that an unnecessary offset (bias) is added to the sensor value or an error can be caused in a gain (sensing sensitivity coefficient) due to a characteristics variation of the sensor (originating in manufacturing tolerance and the like) or aging. Therefore, conventional methods of compensating the error by calculating the gain and the offset have been proposed, for example, as described in JP-A-2002-242750.
The device described in JP-A-2002-242750 obtains outputs Sr1, Sr2 (sensor values) of the cylinder pressure sensor and reference pressure values P1, P2 (references for sensor value correction) at two crank-angle positions in a compression stroke. The device calculates a gain A and an offset B by solving following simultaneous equations (Expression (1)) based on a polytropic change during the compression stroke.P1=A×Sr1+B, P2=A×Sr2+B  Expression (1)
The device corrects the sensor value based on the gain A and the offset B. The device estimates the reference pressure values P1, P2 from an output of an intake pressure sensor outputted during an intake stroke and the like through computation.
Thus, the device can obtain the proper sensor output by correcting the sensor value even when a certain error arises in the sensor value. However, practically, a situation such as complete breakdown of the sensor, which cannot be handled by the correction, can occur. Therefore, it is desirable to perform suitable failsafe processing according to the degree of the abnormality of the sensor. As for this point, the device still has a room for improvement.